Tracking error detection device

ABSTRACT

A tracking error detecting apparatus converts photoelectric current obtained by a photo detector into voltage signals at current/voltage conversion circuits, and adds the voltage signals to generate two signal series at adders. The two signal series are digitalized at analog to digital converters, are subjected to interpolation processing at interpolation filters and have their zero cross points detected by zero cross point detector circuits respectively. A phase difference between the zero cross points of the two signal series is detected by a phase difference detector circuit, and the phase difference is subjected to band restriction by a low pass filter, thereby to obtain a tracking error signal. The tracking error detecting apparatus is capable of coping with speed doubling of an optical recording/reproducing apparatus and density enhancing of an optical recording medium with a small size and at low cost.

TECHNICAL FIELD

The present invention relates to a tracking error detecting apparatuswhich detects a tracking error of an optical spot that is obtained byradiating light on an optical recording medium.

BACKGROUND ART

As a method of obtaining a tracking control signal from an optical disktypified by a CD or a DVD in which information is recorded in the formof concavo-convex pits, a phase difference method has been employed inrecent years.

As disclosed in Japanese Published Patent Application No. Hei.10-162381,the phase difference method is one which obtains a tracking error signalutilizing that when an optical spot deviates from the center of aninformation pit while the optical spot radiated on an informationrecording surface of an optical disk passes traversing on theinformation pit, a mapped image of the information pit on a photodetector (diffraction pattern) varies. That is, when the photo detectoris divided in a track length direction for the information pit mappedimage to see an output signal level according to respective acceptedlight quantities, the way of variation differs according to thedirection and amount of the deviation of the optical spot from thecenter of the information pit. Therefore, by seeing the phase differenceof the binarized signal which is obtained by binarizing the output ofthe photo detector with a prescribed level, a tracking error signalindicating the direction and amount of the deviation of the optical spotcan be obtained.

A conventional method for detecting a tracking error will be describedwith reference to FIGS. 4 to 14.

FIG. 4 is a schematic diagram illustrating a main configuration of anoptical pickup part 100 in FIG. 4, an astigmatic method is employed fora detection of a focus error signal.

A luminous flux radiated from a light source 1, such as a semiconductorlaser is converted into a parallel beam at a collimator lens 3, goesthrough a half mirror 6, is converged by an objective lens 4, and isradiated on an information recording surface 51 on an optical recordingmedium (such as an optical disk) 5 as a small optical spot. A reflectedlight of the optical spot goes through the objective lens 4, has itsoptical path inflected in the right hand direction of the figure by thehalf mirror 6, and reaches a photo detector 2 through a convex lens 61and a cylindrical lens 62 to be a convergent light having two focusescharacteristic in the astigmatic method. Information on the opticalrecording medium 5 is recorded by an information pit line havingunevenness.

Next, a description will be given of a method of obtaining a trackingerror signal which indicates a positional deviation of the optical spotin a vertical direction against the pit line (track) in the informationrecording surface, by utilizing a diffraction pattern of light generatedwhen the optical spot passes traversing on the pit.

An intensity distribution pattern (far field pattern) of the reflectedlight quantity of the optical spot changes according to the position ofan information pit on which the optical spot passes traversing,

FIGS. 5(a)-(c), 6(a)-(c), and 7(a)-(c) are diagrams exemplifying changesof the far field pattern of the reflected light quantity when theoptical spot passes traversing on the pit. (a) of each figure is adiagram illustrating the physical relationship between an optical spot12 and an information pit 13 (the center of the information pit 13 isdescribed by a dotted line), and the optical spot 12 proceeds on theinformation pit 13 in a direction of arrows. (b) of each figure showsthe transition of the intensity distribution pattern (far field pattern)of the reflected light quantity on the photo detector 2, and the threepatterns shown in (b) of each figure respectively represent patternswhen the optical spot 12 is at three positions shown in (a). (c) of eachfigure shows two signals obtained from the photo detector 2. Further,the photo detector 2 has photo acceptance units 2 a-2 d, respective twosbeing arranged vertically and horizontally, and the two signals obtainedin (c) of each figure are ones which are obtained as a result of addingsignals, that are obtained from the four photo acceptance units 2 a-2 d,for the photo acceptance units in a diagonal direction, respectively(i.e., 2 a+2 d and 2 b+2 c).

For example, as shown in FIG. 5(a), when the optical spot 12 passestraversing on the left of the center of the information pit 13 in thedirection of movement, the pattern changes to rotate clockwise as shownin FIG. 5(b), resulting in two signals out of phase as in FIG. 5(c).

As shown in FIG. 6(a), when the optical spot 12 passes traversing on thecenter of the information pit 13, that is, the center of a track, thepattern changes symmetrically as in FIG. 6(b), resulting in two signalsin phase as in FIG. 6(c).

As shown in FIG. 7(a), when the optical spot 12 passes traversing on theright of the center of the information pit 13 in the direction ofmovement, the pattern changes in a counterclockwise direction as shownin FIG. 7(b), resulting in two signals out of phase as in FIG. 7(c).

As described above, it is proved that the transition of the fieldpattern changes when the optical spot deviates from the center of theinformation pit. The phase difference method is the one that utilizesthe changes of the far filed pattern so as to detect a tracking errorsignal. That is, the method comprises comparing phases of two addingsignals obtained from the photo detector 2, and detecting the degree ofphase advancement or delay, thereby recognizing a positional deviationbetween the optical spot 12 and the information pit 13.

A conventional tracking error detecting apparatus will be described withreference to FIGS. 8 and 9(a)-(h). FIG. 8 is a block diagramillustrating an example of a tracking error detecting apparatus whichdetects a phase difference to detect a tracking error signal, and FIGS.9(a)-(h) diagrams of illustrating waveforms of signals denoted by(a)-(h) in FIG. 8. Further, FIGS. 9(a)-(h) diagrams of waveforms in acase where according to a passage of time, the optical spot 12 passestraversing on the information pit 13, crossing from the left side to theright side in the direction of movement, that is, changing from thestate in FIGS. 5(a)-(c) to that in FIGS. 7(a)-(c).

The photo detector 2 has the photo acceptance units 2 a, 2 b, 2 c, and 2d, respective twos being arranged vertically and horizontally, anddetects optical signals to project into respective units as aphotoelectric current. The detected photoelectric current is convertedinto voltage signals by current/voltage conversion circuits 7 a, 7 b, 7c, and 7 d, respectively.

Next, adder 8 a and 8 b adds signals which are obtained from two pairsof units in a diagonal direction of the photo detector 2, for respectivepairs. That is, an adder 8 a adds outputs of the current/voltageconversion circuits 7 a and 7 c, and an adder 8 b adds outputs of thecurrent/voltage conversion circuit 7 b and 7 d. Two adding signals (a)and (b) become waveforms shown in FIGS. 9(a) and 9(b), respectively.

The adding signals (a) and (b) pass through binary circuits 9 a and 9 bso that binary signals (c) and (d) are obtained, respectively.

A phase difference detector circuit 10 detects a phase difference ofrise or fall of the binary signals (c) and (d). In the circuitconfiguration shown in FIG. 8, a phase difference of fall is detectedemploying D-type flip flops (D-FF) 101 a and 101 b. The D-FFs 101 a and101 b have input terminals D, clock input terminals T, reset inputterminals R, and output terminals Q and Q-, and when an input of thereset input terminal R is at logic level, an output of the outputterminal Q is unconditionally at level, and when an input of the resetinput terminal R is at logic level, a signal the logic level of which isthe same as that applied to the input terminal D is outputted from theterminal Q at the fall of the clock input terminal T, “H”→“L”. That is,the D-FFs 101 a and 101 b detect phase differences of the binary signals(c) and (d) to obtain time difference pulses (e) and (f), respectively.The time difference pulse (e) is outputted from the output terminal Q ofthe D-FF 101 a, and the time difference The time difference pulses (e)and (f) are converted into a pulse-width modulation signal (g) at adifference detector 102, which further passes through a low-pass filter11 to be an analog tracking error signal (h).

FIG. 10 illustrates a waveform of the tracking error signal (h) obtainedwhen the tracking error signal is observed for plural tracks. Whenpaying notice to a neighborhood of a specific track, the tracking errorsignal (h) obtained by the tracking error detecting apparatus shown inFIG. 8 is a nearly linear signal which is at a zero level when theoptical spot is on the center of the track, and which, when the opticalspot deviates right and left therefrom, has polarities according to thedirection of the deviation. When observing the tracking error signal forplural tracks, the above-described linear signal waveform appears foreach track, and when the optical spot is between tracks, a zero level isobtained, whereby as a whole, a serrate waveform repeated for each trackis obtained as shown in FIG. 10.

In order to perform a tracking servo control employing the trackingerror signal which appears as a serrate waveform repeatedly for eachtrack with the polarity as in FIG. 10, a tracking servo control systemis constructed so as to drive the objective lens 4 by a means generallyreferred to as a tracking actuator according to the positive andnegative of the tracking error signal.

Further, since the conventional phase difference method detects thetracking error signal from respective pits on which the optical spotpasses traversing, it is likely to be affected by the shape or depth ofthe pit, whereby an offset is generated in the tracking error signalwhen the objective lens 4 is FIGS. 11(a)-(c) and 12(a)-(c) are diagramsillustrating principles of offset generation when detecting the trackingerror signal by the phase difference method, and FIGS. 11(a)-(c) shows acase where the depth of the information pit 13 is λ/4 (λ: wavelength oflight source), while FIGS. 12(a)-(c) shows a case where the depth of theinformation pit is other than λ/4. In the figures, (a) Figure (a)illustrates an intensity distribution pattern (far field pattern) of thereflected light quantity on the photo detector 2 when the objective lens4 does not move, Figure (b) illustrates an intensity distributionpattern (far field pattern) of the reflected light quantity on the photodetector 2 when the objective lens 4 moves, and Figure (c) illustrates atracking error signal obtained. Further, Figures (a) and (b) illustratecases where the optical spot 12 passes traversing on the center of thetrack and is located at the end of the information pit 13.

As shown in FIG. 11(a), in a case where the depth of the information pit13 is λ/4 and the objective lens 4 does not move, patterns which appearin a first area (2 a+2 d) into which the photo acceptance units in adiagonal direction of the photo detector 2, 2 a and 2 d, are combinedand in a second area (2 b+2 c) into which the other photo acceptanceunits in a diagonal direction, 2 b and 2 c, are combined are the same.In addition, as shown in FIG. 11(b), even when the objective lens 4moves and the optical spot 12 on the photo detector 2 moves, the phasedifference between signals outputted from the first area (2 a+2 d) andthe second area (2 b+2 c), respectively is zero, as long as the opticalspot 12 is on the center of the track. Therefore, as shown in FIG.11(c), tracking error signals, the waveform patterns of which at theparts indicated by arrows A and B are the same, can be obtained.

Meanwhile, as shown in FIGS. 12(a)-(c), in a case where the depth of theinformation pit 13 is other than λ/4, a phase difference between signalsoutputted from the first area (2 a+2 d) and the second area (2 b+2 c)may be generated. As shown in FIG. 12(a), when a reflected light of thephoto detector 2 does not move, there is no phase difference between thefirst area (2 a+2 d) and the second area (2 b+2 c), resulting in atracking error signal of zero, but on the other hand, when the objectivelens 4 moves as shown in FIG. 12(b), there is generated an unbalance andthus, the phase difference between the first area (2 a+2 d) and thesecond area (2 b+2 c), resulting in a generation of an offset in atracking error signal. Therefore, tracking error signals, the waveformpatterns of which at the parts indicated by arrows A and B in FIG. 12(c)are different, are obtained. When an offset is generated, it isimpossible to perform tracking toward the center of the track, therebydeteriorating the quality of a reproduced waveform.

To solve the above-mentioned problems, a tracking error detectingapparatus as shown in FIG. 13 is proposed. In FIG. 13, the sameconfigurations as those shown in FIG. 8 are denoted by the samereference numerals.

The above-described tracking error detecting apparatus adjusts thephases of the signals outputted from the photo detector 2 by employingdelay circuits 14 a and 14 b, and thus, the offset of a phase differencetracking error signal can be canceled, thereby performing trackingtoward the center of the track.

However, in case of the tracking error detection by the conventionalmethod, the tracking error signal is detected by an analog signalprocessing, whereby it is not suited for doubling the speed of anoptical recording/reproducing apparatus and for enhancing a density ofan optical recording medium.

Here, problems due to doubling of speed and enhancing of density will bedescribed.

While the tracking error detecting apparatus by an analog signalprocessing shown in FIG. 13 constructs an all pass filter with the delaycircuits 14 a and 14 b for canceling an offset, and obtains the delayamount by group delay of the filter, in a case where the opticalrecording/reproducing apparatus doubles its speed, a channel route ofread data of the optical recording/reproducing apparatus is different,whereby the required amount of delay changes considerably, and thus, theoptimization of the delay circuits is required.

Further, when the recording density of the optical recording medium ishigh, the high frequency component of a read signal obtained from thephoto detector 2 is attenuated, whereby it is impossible to detect aphase difference signal correctly.

As a means to solve this, a tracking error detecting apparatus a asshown in FIG. 14 is proposed. In FIG. 14, the same configurations asthose shown in FIG. 8 are denoted by the same reference numerals, andtheir detailed descriptions will be omitted.

The tracking error detecting apparatus shown in FIG. 14 performs a highfrequency emphasis toward two sum signals of the photo detector 2, (2a+2 d and 2 b+2 c), which are obtained by the adders 8 a and 8 b, bywaveform equalization filters 15 a and 15 b, and binarizes them by thebinary circuits 9 a and 9 b subsequently, so as to obtain a phasedifference signal, whereby attenuation of the high frequency componentdue to high density can be compensated.

However, since the waveform equalization filters 15 a and 15 b arecomposed of analog FIR filters, an all pass filter is required tocompose a delay part of the FIR filter, whereby a problem described inthe above-mentioned speed doubling occurs. In addition, when therecording density is different, the characteristics of high frequencyemphasis required are different, whereby it is impossible to cope by theabove-described tracking error detecting apparatus when further densityenhancing is performed.

As described above, it is difficult to cope with speed doubling in anoptical recording/reproducing apparatus and density enhancing of anoptical recording medium by the conventional tracking error detectingapparatus which performs the tracking error detection by an analogprocessing. Further, since the conventional tracking error detectingapparatus has a large number of configurations involving the analogprocessing, it is difficult to unite the tracking error detectingapparatus with peripheral digital signal processing parts.

The present invention is made to solve the above-mentioned problems andhas for its object to provide a tracking error detecting apparatus whichcan cope with speed doubling of an optical recording/reproducingapparatus and density enhancing of an optical recording medium in asmall size and at low cost.

SUMMARY OF THE INVENTION

A tracking error detecting apparatus according to the present inventioncomprises: a photo detector for receiving reflected light of the opticalspot and outputting photoelectric current according to the photoacceptance quantity; current/voltage conversion circuits for convertingthe photoelectric current of the photo detector into voltage signals;signal generators for generating two signal series, the phases of whichchange each other according to a tracking error of the optical spot,from the voltage signals; analog-digital converters for discretizing thetwo signal series to obtain first and second digital signal series;interpolation filters for performing interpolation processing toward thefirst and second digital signal series respectively; zero cross pointdetector circuits for respectively detecting zero cross points of thefirst and second digital signal series interpolated by the interpolationfilters; a phase difference detector circuit for detecting a phasedifference between the zero cross point of the first digital signalseries and the zero cross point of the second digital signal series; anda low-pass filter for performing band restriction toward the detectedphase difference to obtain a tracking error signal.

With the tracking error detecting apparatus of this configuration, atracking error can be detected by digital signal processing, whereby itis easy to unite the signal processing after the ADC with peripheraldigital signal processing parts. Further, required analog signalprocessing blocks can be reduced considerably. In addition, it ispossible to cope with speed doubling in an optical recording/reproducingapparatus and density enhancing of an optical recording medium, wherebyan optical recording/reproducing apparatus can be provided in a smallsize and at low cost.

Further, another conformation of the tracking error detecting apparatusaccording to the present invention comprises: a photo detector forreceiving reflected light of the optical spot and outputtingphotoelectric current according to the photo acceptance quantity;current/voltage conversion circuits for converting the photoelectriccurrent of the photo detector into voltage signals; analog-digitalconverters for discretizing the voltage signals to convert into digitalsignals; interpolation filters for performing interpolation processingtoward the digital signals; signal generators for generating first andsecond digital signal series, the phases of which change each otheraccording to a tracking error of the optical spot, from the signalsobtained at the interpolation filter; zero cross point detector circuitsfor detecting zero cross points of the first and second digital signalseries respectively; a phase difference detector circuit for comparingphases of the zero cross point of the first digital signal series andthe zero cross point of the second digital signal series, so as todetect a phase difference; a low-pass filter for performing bandrestriction toward the detected phase difference; an offset detectorcircuit for detecting an offset in a tracking error signal from theoutput signal of the low-pass filter; and a factor setting circuit forsetting a factor of the interpolation filter according to the detectedoffset.

With the tracking error detecting apparatus of this configuration, atracking error can be detected by digital signal processing, whereby itis easy to unite the signal processing after the ADC with peripheraldigital signal processing parts. Further, required analog signalprocessing blocks can be reduced considerably. In addition, it ispossible to cope with speed doubling in an optical recording/reproducingapparatus and density enhancing of an optical recording medium, wherebyan optical recording/reproducing apparatus can be provided in a smallsize and at low cost.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating the configuration of a trackingerror detecting apparatus according to a first embodiment of the presentinvention.

FIGS. 2(a)-(c) are explanation diagrams for explaining the operation ofthe tracking error detecting apparatus according to the first embodimentof the invention.

FIG. 3 is a block diagram illustrating the configuration of a trackingerror detecting apparatus according to a second embodiment of thepresent invention. FIG. 4 is a schematic diagram illustrating a mainconfiguration of an optical pickup part 100 in a conventional opticaldisk reproducing apparatus. FIGS. 5(a)-(c) are diagrams exemplifyingchanges of an intensity distribution pattern of the reflected lightquantity when an optical spot passes traversing on a pit.

FIGS. 6(a)-(c) are other diagrams exemplifying changes of an intensitydistribution pattern of the reflected light quantity when an opticalspot passes traversing on a pit.

FIG. 7 is another diagram exemplifying changes of an intensitydistribution pattern of the reflected light quantity when an opticalspot passes traversing on a pit.

FIG. 8 is a block diagram illustrating the configuration of aconventional tracking error detecting apparatus.

FIGS. 9(a)-(h) are diagrams illustrating waveforms of signals denoted by(a)-(h) in FIG. 8.

FIG. 10 is a diagram illustrating a waveform seen when a tracking errorsignal is observed for plural tracks.

FIGS. 11(a)-(c) are diagrams illustrating a principle of offsetgeneration in a case where the depth of a pit 13 is λ/4.

FIGS. 12(a)-(c) are diagrams illustrating a principle of offsetgeneration in a case where the depth of the pit 13 is other than λ/4.

FIG. 13 is a block diagram illustrating the configuration of anotherconventional tracking error detecting apparatus.

FIG. 14 is a block diagram illustrating the configuration of anotherconventional tracking error detecting apparatus.

DETAILED DESCRIPTION OF THE INVENTION Embodiment

Hereinafter, a tracking error detecting apparatus according to a firstembodiment will be described with reference to FIGS. 1 and 2(a)-(c). Anexample of an optical pickup part of an optical disk reproducingapparatus which is provided with the tracking error detecting apparatusaccording to the first embodiment is the optical pickup part describedin FIG. 4 as a conventional

FIG. 1 is a block diagram illustrating the configuration of a trackingerror detecting apparatus 1 according to the first embodiment. The sameconfigurations as those shown in FIG. 8 are denoted by the samereference numerals, and their detailed descriptions will be omittedhere.

The tracking error detecting apparatus 1 comprises a photo detector 2which has photo acceptance units 2 a-2 d that accept a reflected lightof an optical spot, and outputs photoelectric current according to thephoto acceptance quantity of the photo acceptance units, first to fourthcurrent/voltage conversion circuits 7 a-7 d which convert thephotoelectric current output of the photo detector 2 into voltagesignals, signal generators, i.e., first and second adders 8 a and 8 bwhich generate two signal series, the phases of which change from eachother, from the voltage signal obtained at the first to fourthcurrent/voltage conversion circuits 7 a-7 d according to a trackingerror of the optical spot, first and second analog-digital converters(ADC) 16 a and 16 b which obtain first and second digital signal seriesfrom the two signal series, first and second interpolation filters 17 aand 17 b which perform an interpolation processing toward the first andsecond digital signal series respectively, first and second zero crosspoint detector circuits 18 a and 18 b which respectively detect zerocross points of the first and second digital signal series interpolatedby the first and second interpolation filters 17 a and 17 b, a phasedifference detector circuit 19 which detects a phase difference betweenthe zero cross point of the first digital signal series and the zerocross point of the second digital signal series, and a low-pass filter(LPF) 11 which performs a band restriction toward a phase differencesignal to obtain a tracking error signal.

The photo detector 2 is provided with the photo acceptance units 2 a, 2b, 2 c, and 2 d, which are divided into four parts in the shape of asquare with a cross inside, for example, as the photo acceptance unitsand receives a reflected light of the optical spot which is obtained byradiating light on a track of an optical recording medium (not shown)and outputs photoelectric current according to The first to fourthcurrent/voltage conversion circuits 7 a, 7 b, 7 c, and 7 d convert thephotoelectric current as an output of the photo detector 2 into voltagesignals for respective photo acceptance units 2 a, 2 b, 2 c, and 2 d.

The first adder 8a adds outputs of the first and third current/voltageconversion circuits 7 a and 7 c, and the second adder 8 b adds outputsof the second and fourth current/voltage conversion circuits 7 b and 7d.

The first and second ADCs 16 a and 16 b subject discretization(sampling) to the signal series outputted from the first and secondadders 8 a and 8 b, respectively, to obtain the first and second digitalsignal series.

The interpolation filters 17 a and 17 b obtain interpolation databetween sampling data of the digital signal series obtained by the firstand second ADCs 16 a and 16 b, and as a method of interpolation, Nyquistinterpolation, for example, is conceivable.

The zero cross point detector circuits 18 a and 18 b detect zero crosspoints at rise or fall of the two interpolated data series. As a methodfor detecting a zero cross point, one which obtains a change point of acode of the interpolated data series (+→−, or −→+), for example, isconceivable.

Next, the operation of the phase difference detector circuit 19 will bedescribed with reference to FIGS. 2(a)-(c). FIG. 2(a) shows an exampleof the first signal series outputted from the first zero cross pointdetector circuit 18 a, FIG. 2(b) shows an example of the second signalseries outputted from the second zero cross point detector circuit 18 b,and FIG. 2(c) shows a phase difference signal obtained by the phasedifference detector circuit 19. A description will be given of symbolsused in the data series (a) and (b) FIG. 2. A mark of ∘ indicates thesampling data obtained by the first or second ADC 16 a or 16 b, a markof Δ indicates the interpolation data series obtained by the first orsecond interpolation filter 17 a or 17 b, and marks of  and ▴ indicatezero cross points obtained from the sampling data series and theinterpolation data series. Further, the phase difference signaldescribed in FIG. 2(c) is one when paying notice to a neighborhood of aspecific track, and which is obtained at a fall of the two data series,the phase difference between which is to be obtained. Further, thenumber of the interpolation data is n=3.

The phase difference detector circuit 19 detect a phase differencesignal from the distance of the zero cross points in respectivewaveforms of the first and second signal series. When comparing the zerocross points of the first signal series (a) and the second signal series(b), it proves that the amount of the phase difference obtained isproportional to the distance of the zero cross points of the two signalseries (a) and (b). Further, a direction of phase lag is obtained byjudging at which points zero crossing is performed first between thezero cross points of the two signal series (a) and (b). From the amountof phase difference and a direction of the phase lag detected asdescribed above, the phase difference signal shown in (c) can beobtained.

The so-obtained phase difference signal is a nearly linear signal, whenpaying notice to a neighborhood of a specific track. When observing thephase difference signal for plural tracks, a nearly serrate waveformrepeated for each track can be obtained as a whole as described in FIG.10.

The phase difference signal detected at the phase difference detectorcircuit 19 is finally subjected to a band restriction by the LPF 11 toobtain a tracking error signal having a band required for a trackingservo control.

Further, a factor setting circuit (not shown) which can set factors ofthe interpolation filters 17 a and 17 b may be provided. In this case,when a new factor in which a filter factor having characteristics ofhigh frequency emphasis is folded in a filter factor for interpolationis set as the factor of the interpolation filters 17 a and 17 b,“interpolation to obtain a tracking error signal” and “filtering tocompensate for an attenuation of a high frequency component accompanyinghigh density” can be performed at the same time with one filter, therebyconsiderably reducing the circuit scale.

As described above, the tracking error detecting apparatus 1 accordingto the first embodiment can detect a tracking error by a digital signalprocessing, whereby it is possible to cope with speed doubling of anoptical recording/reproducing apparatus and density enhancing of anoptical recording medium, which a conventional tracking error detectionby an analog signal processing cannot cope with.

Further, since the processing after the first and second ADCs 16 a and16 b is a digital signal processing, the configuration after the firstand second ADCs 16 a and 16 b can be united with digital signalprocessing parts around the tracking error detecting apparatus 1 easily.In addition, the configuration involving an analog signal processing canbe considerably reduced, thereby realizing an opticalrecording/reproducing apparatus being downsized and low in cost.

Embodiment 2

Next, a second embodiment will be described with reference to FIG. 3.FIG. 3 is a block diagram illustrating the configuration of a trackingerror detecting apparatus 30 according to the second embodiment. Thesame configurations as those shown in FIG. 1 are denoted by the samereference numerals, and their detailed descriptions will be omitted.

A tracking error detecting apparatus according to a second embodimentconverts photoelectric current obtained at the photo detector 2 intovoltage, digitalizes it, and performs an interpolation processing towardit, thereby to generate two signal series, the phases of which changeaccording to a tracking error. Further, according to an offset detectedfrom a tracking error signal as an output of the LPS 11, the position ofdata interpolated by the interpolation filters can be controlled.

The tracking error detecting apparatus 30 comprises a photo detector 2which has photo acceptance units 2 a, 2 b, 2 c, and 2 d, and obtains aphotoelectric current output, first to fourth current/voltage conversioncircuits 7 a, 7 b, 7 c, and 7 d which convert the photoelectric currentinto voltage signals for respective photo acceptance units 2 a, 2 b, 2c, and 2 d, first to fourth ADCs 16 e, 16 f, 16 g, and 16 h whichsubject discretization (sampling) to the signals obtained by the firstto fourth current/voltage conversion circuits 7 a, 7 b, 7 c, and 7 d, soas to convert the signals into digital signals, first to fourthinterpolation filters 17 e, 17 f, 17 g, and 17 h which obtaininterpolation data among sampling data in four discretized signalseries, first and second adders 8 a and 8 b as signal generators whichgenerate two signals, i.e., first and second digital signal series forperforming phase comparison from the four interpolated data series,first and second zero cross point detector circuits 18 a and 18 b whichrespectively detect zero cross points of two signals obtained by thefirst and second adders 8 a and 8 b, a phase difference detector circuit19 which detects a phase difference signal from the signals outputtedfrom the first and second zero cross point detector circuits 18 a and 18b, a low-pass filter (LPF) 11 which obtains a tracking error signal, anoffset detector circuit 20 which detects an offset in the tracking errorsignal from an output signal of the LPF 11, and a factor setting circuit21 which set factors of the interpolation filters 17 e, 17 f, 17 g, and17 h according to the detected offset.

As a method of offset detection performed at the offset detector circuit20, one which performs detection by comparing peak values of + side andside in the tracking error signal, for example, is conceivable. Theoffset detected by the offset detector circuit 20 is inputted to thefactor setting circuit 21. According to the detected offset, the factorsetting circuit 21 adjusts factors of the first and second interpolationfilters 17 e and 17 f and factors of the third and fourth interpolationfilters 17 g and 17 h, and deviates data position to interpolate, so asto cancel the offset in the tracking error signal.

For example, in a case where the betweenness of sampling data of T rateis the number of interpolation data n=3 so as to perform interpolation,and a factor to obtain interpolation data at intervals of T/4 is set,when the factor which is made by adding offset to the first and secondinterpolation filters 17 e and 17 f at intervals of T/4 (this offset isΔ (delta)) is set, the interpolation data series which is delayed oradvanced for the time of the offset Δ can be obtained.

As described above, the offset can be canceled only by changing thefactors of the interpolation filters 17 e, 17 f, 17 g, and 17 h, wherebyit is possible to cope with speed doubling of an opticalrecording/reproducing apparatus.

Further, in a case where factors of the interpolation filters 17 e, 17f, 17 g, and 17 h are set at the factor setting circuit 21, by setting anew factor which is obtained by folding a filter factor havingcharacteristics of high frequency emphasis in a filter factor to performinterpolation, “interpolation to obtain a tracking error signal”, “delayto cancel an offset”, and “filtering to compensate for an attenuation ofhigh frequency component accompanying high density” can be performed atthe same time with one filter, thereby considerably reducing the circuitscale, resulting in an optical recording/reproducing apparatus providedwith a small size and at low cost. Further, while as the photo detector2, the one which has the four units 2 a, 2 b, 2 c, and 2 d, respectivetwos being arranged vertically and horizontally, is employed in thefirst and second embodiments for simplifying descriptions, a photodetector is not restricted to the above-described example. In a casewhere as a photo detector, one which has a different configuration fromthat of the above-mentioned example is employed, while the configurationafter the current/voltage conversion circuit is modified according tothe configuration of the photo detector, the configuration after beingmodified and a method of modification can be realized easily by thoseskilled in the art. Therefore, it goes without saying that a trackingerror detecting apparatus modified according to a photo detector shouldbe included in the scope of the present invention.

APPLICABILITY IN INDUSTRY

As described above, the tracking error detecting apparatus according tothe present invention can considerably reduce the circuit scale, canrealize an optical recording/reproducing apparatus which reproduces anoptical recording medium as typified by a CD or a DVD in a small sizeand at low cost, and particularly, cope with speed doubling of anoptical recording/reproducing apparatus and deisity enhancing of anoptical recording medium.

What is claimed is:
 1. A tracking error detecting apparatus fordetecting a tracking error of an optical spot obtained by radiatinglight on an optical recording medium, said tracking error detectingapparatus comprising: a photo detector for receiving reflected light ofthe optical spot and outputting photoelectric current according to aphoto acceptance quantity; a plurality of current/voltage conversioncircuits for converting the photoelectric current of the photo detectorinto voltage signals; a plurality of signal generators for generatingtwo signal series, phases of the two signal series changing with respectto each other according to a tracking error of the optical spot, fromthe voltage signals; a plurality of analog-digital converters fordiscretizing the two signal series to obtain first and second digitalsignal series; a plurality of interpolation filters for performinginterpolation processing of the first and second digital signal series,respectively; a plurality of zero cross point detector circuits forrespectively detecting zero cross points of the first and second digitalsignal series interpolated by the interpolation filters; a phasedifference detector circuit for detecting a phase difference between thezero cross point of the first digital signal series and the zero crosspoint of the second digital signal series; and a low-pass filter forperforming band restriction toward the detected phase difference toobtain a tracking error signal.
 2. A tracking error detecting apparatusfor detecting a tracking error of an optical spot obtained by radiatinglight on an optical recording medium said tracking error detectingapparatus comprising: a photo detector for receiving reflected light ofthe optical spot and outputting photoelectric current according to aphoto acceptance quantity; a plurality of current/voltage conversioncircuits for converting the photoelectric current of the photo detectorinto voltage signals; a plurality of analog-digital converters fordiscretizing the voltage signals for conversion into digital signals; aplurality of interpolation filters for performing interpolationprocessing toward the digital signals; a plurality of signal generatorsfor generating first and second digital signal series, phases of thefirst and second digital signal series changing with respect to eachother according to a tracking error of the optical spot, from signalsobtained at the interpolation filters; a plurality of zero cross pointdetector circuits for detecting zero cross points of the first andsecond digital signal series, respectively; a phase difference detectorcircuit for comparing phases of the zero cross point of the firstdigital signal series and the zero cross point of the second digitalsignal series, so as to detect a phase difference; and a low-pass filterfor performing band restriction toward the detected phase difference. 3.The tracking error detecting apparatus as defined in claim 1, whereinthe photo detector has a plurality of photo acceptance units, respectivepairs of the photo acceptance units being arranged vertically andhorizontally, and the signal generators are each provided with an adderwhich respectively adds signals obtained from the photo acceptance unitsin a diagonal direction among the signals obtained from the photoacceptance units, so as to obtain the two signal series.
 4. The trackingerror detecting apparatus as defined in claim 1, further comprising: afactor setting circuit for setting a factor of the interpolation filtersto a desired value.
 5. The tracking error detecting apparatus as definedin claim 4, further comprising: an offset detector circuit for detectingan offset in the tracking error signal outputted from the low-passfilter, wherein the factor setting circuit sets the factor according tothe detected offset so that data in place are interpolated by theinterpolation filters.
 6. The tracking error detecting apparatus asdefined in claim 4, wherein the factor set by the factor setting circuitis one in which a factor to perform interpolation and a factor tocompensate for an attenuation of a high frequency component are folded.7. The tracking error detecting apparatus as defined in claim 2, whereinthe photo detector has a plurality of photo acceptance units, respectivepairs of the photo acceptance units being arranged vertically andhorizontally, and the signal generators are each provided with an adderwhich respectively adds signals obtained from the photo acceptance unitsin a diagonal direction among the signals obtained from the photoacceptance units, so as to obtain the first and second signal series. 8.The tracking error detecting apparatus as defined in claim 7, furthercomprising: a factor setting circuit for setting a factor of theinterpolation filters to a desired value.
 9. The tracking errordetecting apparatus as defined in claim 8, further comprising: an offsetdetector circuit for detecting an offset in the tracking error signaloutputted from the low-pass filter, wherein the factor setting circuitsets the factor according to the detected offset so that data in placeare interpolated by the interpolation filters.
 10. The tracking errordetecting apparatus as defined in claim 9, wherein the factor set by thefactor setting circuit is one in which a factor to perform interpolationand a factor to compensate for an attenuation of a high frequencycomponent are folded.
 11. The tracking error detecting apparatus asdefined in claim 8, wherein the factor set by the factor setting circuitis one in which a factor to perform interpolation and a factor tocompensate for an attenuation of a high frequency component are folded.12. The tracking error detecting apparatus as defined in claim 2,further comprising: a factor setting circuit for setting a factor of theinterpolation filters to a desired value.
 13. The tracking errordetecting apparatus as defined in claim 12, further comprising: anoffset detector circuit for detecting an offset in the tracking errorsignal outputted from the low-pass filters, wherein the factor settingcircuit sets the factor according to the detected offset so that data inplace are interpolated by the interpolation filters.
 14. The trackingerror detecting apparatus as defined in claim 13, wherein the factor setby the factor setting circuit is one in which a factor to performinterpolation and a factor to compensate for an attenuation of a highfrequency component are folded.
 15. The tracking error detectingapparatus as defined in claim 12, wherein the factor set by the factorsetting circuit is one in which a factor to perform interpolation and afactor to compensate for an attenuation of a high frequency componentare folded.
 16. The tracking error detecting apparatus as defined inclaim 5, wherein the factor set by the factor setting circuit is one inwhich a factor to perform interpolation and a factor to compensate foran attenuation of a high frequency component are folded.
 17. Thetracking error detecting apparatus as defined in claim 3, furthercomprising: a factor setting circuit for setting a factor of theinterpolation filters to a desired value.
 18. The tracking errordetecting apparatus as defined in claim 17, further comprising: anoffset detector circuit for detecting an offset in the tracking errorsignal outputted from the low-pass filter, wherein the factor settingcircuit sets the factor according to the detected offset so that data inplace are interpolated by the interpolation filters.
 19. The trackingerror detecting apparatus as defined in claim 18, wherein the factor setby the factor setting circuit is one in which a factor to performinterpolation and a factor to compensate an attenuation of highfrequency component are folded.
 20. The tracking error detectingapparatus as defined in claim 17, wherein the factor set by the factorsetting circuit is one in which a factor to perform interpolation and afactor to compensate an attenuation of high frequency component arefolded.